Lymphocyte activation antigens and antibodies thereto

ABSTRACT

HB15-related lymphocyte activation antigens, and nucleic acid sequences encoding HB15-related antigens are disclosed. Also disclosed are antibodies reactive with HB15.

GOVERNMENT RIGHTS

Part of the work leading to this invention was made with United States Government funds. Therefore, the U.S. Government has certain rights in this invention.

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 08/233,005, filed Apr. 25, 1994, now U.S. Pat. No. 5,710,262, issued Jan 20, 1998, which is a continuation-in-part of application Ser. No. 07/870,029, filed Apr. 17, 1992, now U.S. Pat. No. 5,316,920, issued May 31, 1994.

FIELD OF THE INVENTION

This invention relates to nucleic acid sequences encoding human lymphocyte activation antigens, particularly to sequences encoding lymphocyte activation antigen HB15, and to the proteins and polypeptides encoded by those sequences.

BACKGROUND OF THE INVENTION

The Ig gene superfamily, described by Williams et al., Annu. Rev. Immunol., 88:381-405 (1988), encompasses a large family of genes that are presumed to have evolved from a common precursor. Many of the Ig superfamily members are involved in cell-cell adhesion and signal transduction. In addition, many of the cell-surface molecules which regulate immune responses contain conserved structural features similar to those found in immunoglobulin (Ig). While most members of the Ig gene superfamily contain multiple linearly-assembled Ig-like domains, several proteins have been identified that contain single Ig-like domains.

Single Ig-like domain proteins that are known or assumed to be involved in cell-cell adhesion include: CD8α (Littman et al., Cell 40:237 (1985)), CD8β (Johnson et al., Nature 323:74 (1986)), CD7 (Aruffo et al., EMBO J. 6:3313 (1987)), Thy-1 (Williams et al., Science 216:696 (1982)), CD28 (Aruffo et al., Proc. Natl. Acad. Sci. USA 84:8573 (1987)), CTLA-4 (Brunet et al., Nature 328:267 (1987)) and Po which is a structural protein of the peripheral myelin sheath (Lemke et al., Cell 40:501 (1985)). In addition, other single Ig-domain containing proteins associate with the antigen receptors of B and T lymphocytes, forming multimeric signal-transducing complexes. These include: CD3 γ, δ and ε chains (Gold et al., Nature 321:431-434 (1986); van den Elsen et al., Nature 312:413-418 (1984)), CD79β (Hermanson et al., Proc. Natl. Acad. Sci., USA 85:6890 (1988)), and CD79α (Sakaguchi et al., EMBO J. 7:3457-3464 (1988)).

Two proteins containing single Ig-like domains found on lymphocytes are preferentially associated with cellular activation and are known to be involved in mediating cell-cell interactions. CD28 is expressed much more on activated than nonactivated T and B lymphocytes (Turka et al., J. Immunol. 144:1646 (1990)), and CTLA-4 is expressed mostly, if not exclusively, by activated T and B lymphocytes (Brunet et al., Nature 328:267 (1987); Harper et al., J. Immunol. 147:1037-1044 (1991)). The role of CD28 as a T cell receptor for the CD80 molecule expressed by activated B cells has been recently identified (Linsley et al., Proc. Natl. Acad. Sci. USA 87:5031-503 (1990); Freeman et al., J. Immunol. 143:2714-2722 (1989)), as has a similar role for CTLA-4 (Linsley et al., J. Exp. Med. 174:561-569 (1991)). As with CD28 and CD80, most of the Ig-like domain-containing receptors interact with members of the Ig superfamily present on other cells.

It is an object of the invention to provide a new member of the Ig gene superfamily. Another object of the invention is to provide a protein that is found predominantly on lymphoid tissue. Yet another object of the invention is to provide a protein that contains an extracellular single Ig-like domain. Yet another object of the invention is to provide a nucleic acid encoding the protein or a biologically active portion of the protein. Another object of the invention is to provide nucleic acid probes for identifying the protein or homologs thereof. Yet another object of the invention is to provide an antigen that is present on activated lymphocytes, but absent on inactive lymphocytes and most other cell types.

SUMMARY OF THE INVENTION

The invention is based on the discovery of a human lymphocyte cDNA which encodes a novel glycoprotein present on activated lymphocytes, termed HB15 or CD83 (WHO nomenclature).

The invention thus features a nucleic acid isolate encoding the polypeptide HB15 and able to hybridize to a nucleic acid encoding a polypeptide having an amino acid sequence shown in SEQ ID NO:2. HB15 mammalian analog refers to a polypeptide which has a tissue distribution similar to human HB15, i.e., is present on activated lymphocytes and dendritic cells, and is encoded by a nucleic acid able to hybridize to a nucleic acid encoding the amino acid sequence shown in SEQ ID NO:2. "HB15 fragment" or "HB15 analog fragment" refers to a polypeptide of at least 5 amino acids, preferably at least 10 amino acids, and most preferably at least 20 amino acids, which in its native context is part of a protein having the tissue distribution pattern of HB15. An HB15 fragment or HB15 analog fragment will include a portion of HB15 such as one of the extracellular, transmembrane or cytoplasmic domains, or a smaller polypeptide, such as an immunogenic region of HB15.

In preferred embodiments, the nucleic acid isolate encodes a polypeptide that is recognized by a monoclonal antibody specific for an HB15 epitope. Preferably, the nucleic acid isolate encodes a polypeptide having the complete amino acid sequence shown in SEQ ID NO:2, or the portion of SEQ ID NO:2 comprising the HB15 extracellular domain (i.e., amino acid numbers 1-125), the transmembrane domain (i.e., amino acid numbers 126-147), or the cytoplasmic domain (i.e., amino acid numbers 148-186). The boundaries of the mouse domains are approximately the same as those of the human domains, provided the sequences are aligned as shown in FIG. 6. Preferably, for polynucleotides greater than about 50 bases, the nucleic acid isolate is hybridizable under stringent conditions to a portion of the nucleic acid sequence of SEQ ID NO: 1. For oligonucleotides less than about 50 nucleotides in length, the nucleic acid isolate is hybridizable under low stringency conditions, i.e., at about 42° C. in the presence of 30% formamide according to conditions described in Benton and Davis (1977, Science 196:180), hereby incorporated by reference. Preferably, the nucleic acid isolate is greater than about 15 nucleotides, more preferably greater than about 20, 50 or 100 nucleotides.

The invention also encompasses replicable expression vectors containing nucleic acid sequences encoding the HB15 protein or portions thereof, including an HB15 domain, as defined above, or immunogenic fragments, and host cells transfected with such a vector (e.g., for a bacterial, yeast, or eucaryotic cell culture).

The invention also encompasses HB15 or portions thereof which are immunogenic, and thus useful as immunogens in order to raise antibodies against HB15 or portions thereof including any of its specific domains or fragments thereof.

The invention also features antibodies reactive with HB15 or fragments thereof.

The invention also features methods of producing human HB15 or a mammalian homolog of human HB15, comprising transforming a host cell with a nucleic acid encoding a polypeptide able to hybridize to a sequence encoding the amino acid sequence shown in SEQ ID NO: 2, culturing the transformed cell, and recovering the HB15 protein or homolog from the cell culture.

The invention also encompasses methods of detecting the presence of human HB15 or of a mammalian HB15 analog on a cell, comprising subjecting a cell suspected of bearing HB15 on its surface to an antibody that recognizes HB15, and detecting binding of the antibody to the cell.

The invention also features methods of producing a polypeptide encoded by a nucleic acid isolate greater than about 15 bp and capable of hybridizing under low or high stringency conditions to a nucleic acid sequence shown in SEQ ID NO: 1. The method includes the steps of (a) providing cells which in the untransfected form do not express a nucleic acid isolate greater than about 15 bp and hybridizable to a nucleic acid sequence shown in SEQ ID NO: 1; (b) transfecting the cells with the nucleic acid isolate operably linked to suitable control sequences under conditions effective for the production of the encoded polypeptide; and (c) recovering the polypeptide.

The invention thus also features a polypeptide having HB15 biological activity and encoded by a nucleic acid isolate able to hybridize under low or high stringency conditions to a nucleic acid encoding a polypeptide having the amino acid sequence shown in SEQ ID NO: 2. In addition, the invention includes a polypeptide encoded by a nucleic acid isolate greater than about 15 nucleotides, hybridizable under low or high stringency conditions to the complement of the nucleic acid sequence shown in SEQ ID NO: 1.

The invention also features a purified nucleic acid molecule encoding an amino acid sequence of an HB15 molecule from an animal species other than human, the nucleic acid molecule being isolated by: (1) hybridizing a nucleic acid isolate with a population of nucleic acid molecules from an animal species other than human, preferably under low stringency hybridization conditions, wherein the nucleic acid isolate encodes HB15 or a portion thereof that is recognizable by a monoclonal antibody specific for an HB15 determinant, and is able to hybridize under stringent conditions to a nucleic acid encoding a polypeptide having the amino acid sequence shown in SEQ ID NO: 2 ; (2) identifying a first nucleic acid molecule to which the nucleic acid isolate stringently hybridizes; and (3) isolating the first nucleic acid molecule, wherein the first nucleic acid molecule encodes a polypeptide having an amino-acid sequence shown in SEQ ID NO. 2.

This purified nucleic acid molecule may be further isolated by the additional steps of: (4) hybridizing a nucleic acid isolate with a population of nucleic acid molecules from an animal species other than human wherein said nucleic acid isolate encodes HB15 or is recognizable by a monoclonal antibody specific for an HB15 determinant, and is able to hybridize to a nucleic acid encoding a polypeptide having the amino acid sequence shown in SEQ ID NO: 2; (5) identifying a second nucleic acid molecule to which the nucleic acid isolate hybridizes; and (6) isolating the second nucleic acid, wherein the first and second nucleic acid molecules, joined together in an amino acid reading frame, encode an amino acid sequence of SEQ ID NO. 2.

Preferably, the nucleic acid molecule is a murine nucleic acid.

The invention also features an isolated nucleic acid able to hybridize to the nucleic acid molecule described immediately above, and polypeptides encoded by that nucleic acid molecule.

As used herein the term "identify" is intended to include techniques that require detection, isolation or purification of HB15 protein or its encoding genetic material. The terms "isolated" and "essentially purified" refer to a nucleic acid or protein sequence that has been separated or isolated from the environment in which it was prepared or in which it naturally occurs.

Nucleic acid or protein sequences may be in the form of chimeric molecules, i.e., which lack one or more of the three domains found in the native molecule, or chimeric hybrids in which one domain is substituted with a domain from another type of molecule, e.g., a toxin or an Ig molecule. Examples of chimeric hybrids include but are not limited to molecules which contain extracellular domains in which one or more of these domains are heterologous. Such hybrids, e.g., an immunoglobulin fusion protein, are useful for promoting serum half-life or multimerization of the molecule to increase avidity. Truncated HB15 molecules include but are not limited to HB15 comprising an extracellular domain free of transmembrane and cytoplasmic domains, which is useful for identifying a ligand or disrupting cell/cell interaction, e.g., dendritic/T cell interactions.

The term "immunogenic fragment" refers to a fragment of HB15 that reacts with antibodies specific for a determinant of HB15.

The HB15 protein or immunogenic fragment can be used as antigenic reagents for immunization of a host animal in the preparation of antibodies specific for HB15. An HB15 antibody may also be used to deliver drugs, toxins, or imaging agents to cells that express HB15. HB15 cDNA can be used to produce these proteins or peptide fragments; to identify nucleic acid molecules encoding related proteins or polypeptides (e.g., homologous polypeptides from related animal species and heterologous molecules from the same species); or to construct genes encoding other new, chimeric molecules. In addition, HB15 cDNA can be used to synthesize antisense oligonucleotides for inhibiting the expression of the HB15 protein. Assays for HB15 production or expression by cells are made possible by the development of monoclonal antibodies selectively reactive with the HB15 protein.

Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the structure of the HB15 cDNA clone and the location of restriction sites, showing the extracellular domain ("extracell"), the transmembrane domain ("TM"), and the 3' untranslated region (3'UT);

FIG. 2 shows the cDNA nucleotide sequence (SEQ ID NO:1) and the deduced amino acid sequence (SEQ ID NO:2) of HB15; the vertical arrow represents the predicted cleavage site for generation of the mature protein; numbers shown above the amino acid sequence designate amino acid residue positions of the putative mature protein; numbers to the right of the nucleotide sequence designate nucleotide positions; the * indicates the translation termination codon; underlined nucleotides delineate translated regions with hydrophobic character; underlined amino acids indicate potential N-linked glycosylation attachments sites; wavy underlining delineates a poly (A) attachment signal sequence; amino acids conserved in Ig-like domains are indicated by (+); cysteine residues are circled; arrowheads below the nucleotide sequence denote exon/intron boundaries;

FIG. 3 shows a hypothetical model for the structure of the extracellular domain of HB15, cysteine residues are shown as filled in circles; amino acids encoded by different exons are indicated by alternatively shaded circles; numbers represent the predicted amino acid residue positions as shown in FIG. 2;

FIG. 4A shows immunofluorescence results obtained with three lymphoblastoid cell lines that express HB15 (A) with blood lymphocytes before and after mitogen activation (B); open histograms show cellular reactivity with the HB15a antibody; shaded histograms represent background levels of immunofluorescence staining obtained with unreactive control antibodies;

FIG. 4B shows immunofluorescence results obtained with blood lymphocytes before and after mitogen activation (B), with open and shaded histograms represented as in FIG. 4A;

FIG. 5A shows immunohistochemical analysis of HB15 expression in tonsil and lymph node cells;

FIG. 5B shows immunohistochemical analysis of HB15 expression in germinal centers;

FIG. 5C shows immunohistochemical analysis of HB15 expression in interfollicular regions (i.e., the T-cell zone);

FIG. 5D shows immunohistochemical analysis of CD1 expression in a subpopulation of dendritic cells;

FIG. 5E shows immunohistochemical analysis of HB15 expression in a subpopulation of thymic medulla cells; and

FIG. 5F shows immunohistochemical analysis of HB15 expression in a subpopulation of dendritic cells (skin Langerhan's cells).

FIG. 6 is a comparison of human and mouse CDNA sequences encoding HB15.

FIG. 7 presents sequence locations of oligonucleotide probes used for PCR amplification of human and mouse HB15 cDNAs relative to the human and mouse HB15 DNA sequences.

FIG. 8A shows results of PCR amplification and gel electrophoresis of amplified fragments.

FIG. 8B shows results of Southern blots of the gels shown in FIG. 8A using a probe from the HB15 transmembrane domain.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The lymphocyte activation antigen, HB15, is expressed virtually exclusively by lymphoid tissue and skin Langerhans cells. HB15 is a single-chain cell-surface glycoprotein of M_(r) 45,000. Referring to FIG. 1, the structural features of the HB15 protein, predicted from nucleotide sequence derived from multiple cDNA clones, establish it as a new member of the Ig superfamily. The predicted structure of HB15 is that of a typical membrane glycoprotein with a single extracellular Ig-like domain, a transmembrane domain and an approximately 40 amino acid cytoplasmic domain. cDNA cloned from a human lymphocyte library were analyzed and shown to encode the novel cell-surface glycoprotein HB15, expressed by activated lymphocytes. The mature 186 amino acid protein encoded by the cDNA was composed of a single extracellular V type immunoglobulin (Ig)-like domain, a transmembrane domain and a 39 amino acid cytoplasmic domain. Northern blot analysis revealed that HB15 derives from three mRNA transcripts of ˜1.7, 2.0 and 2.5 kb expressed by lymphoblastoid cell lines. It is likely that the entire coding region for HB15 was identified, as transfection of cell lines with the pHB15 cDNA generated cell surface expression of the protein and the M_(r) of the immunoprecipitated protein was similar in both cDNA transfected cells (˜45,000) and HB15⁺ Raji cells (˜40,000). It is also likely that HB15 undergoes extensive post-translational processing, as HB15 was expressed as a single chain molecule, yet the determined M_(r) was twice the predicted size of the core protein. Since HB15 was also expressed on the surface of cDNA transfected cells, including COS cells, CHO cells, a mouse pre-B cell line and a human erythroleukemia line, it is likely that surface expression is not dependent on expression of other components of a molecular complex as occurs with the Ig-like proteins that associate with the T and B cell antigen receptors.

Monoclonal antibodies reactive with HB15 were produced and used to show that HB15 expression is specific for lymphoblastoid cell lines and mitogen-activated lymphocytes; HB15 was not expressed at detectable levels by circulating leukocytes. Immunohistological analysis revealed that HB15 has a unique pattern of expression among tissues, being found predominantly in hematopoietic tissues with scattered expression by interfollicular cells and weak expression by mantle zone and germinal center cells. Uniquely, HB15 is also expressed by Langerhans cells within the skin and circulating dendritic cells. Thus, the HB15 glycoprotein represents a new member of the Ig superfamily.

Comparison of the HB15 amino acid sequences (SEQ ID NO:2) with other previously identified proteins did not reveal any striking homologies, except the similarity of the extracellular Ig-like domain with other members of the Ig superfamily. The HB15 Ig-like domain contained many of the conserved features found in the V-set of domains, as shown in FIG. 2 (Williams et al., Ann. Rev. Immunol. 88:381-405 (1988)). Based on the homology with Ig domains, HB15 is likely to possess a disulfide bond linking Cys 16 and Cys 88. This would place 71 amino acids between the two Cys residues which is of the appropriate size for V-related domains (Williams et al., supra). There is the potential for additional disulfide bond formation between residues at positions 8, 81 and 110 since these Cys are present in the extracellular domain as well. In addition, HB15 has a Cys residue located within the predicted membrane spanning domain at position 144. Cys residues are also located at identical positions in CD3δ and CD7, suggesting some functional significance, perhaps as sites for fatty acylation (Kaufman et al., J. Biol. Chem. 259:7230-7238, (1984); Rose et al., Proc. Natl. Acad. Sci., USA 81:2050-2054 (1984)). The HB15 cytoplasmic tail is similar in size to that of CD7 (Aruffo et al., EMBO J. 6:3313 (1987)), but shared no amino acid sequence similarity with known proteins. However, the five Ser/Thr residues within this domain could serve as potential sites of phosphorylation. Thus, HB15 appears to be a newly described lymphocyte cell surface antigen that shares no apparent relatedness with previously described structures.

The HB15 extracellular domain is different from the typical Ig-like domain in that it is encoded by at least two exons. Analysis of partial genomic DNA sequence revealed that half of the Ig-like domain is encoded by a single exon and the putative membrane spanning domain is also encoded by a distinct exon (FIG. 2). That Ig-like domains can be encoded by more than one exon has been observed for some members of the Ig superfamily, including the Po protein (Lemke et al., Neuron 1:73-83 (1988)), CD4 (Littman et al., Nature 325:453-455 (1987)) and N-CAM (Owens et al., Proc. Natl. Acad. Sci., USA 84:294-298 (1987)). This finding supports structural analyses which suggested that Ig domains may have arisen from an ancestral half-domain that evolved through duplication and subsequent adjoining. However, each of the above genes and the HB15 gene contain introns at different locations between the sequences coding for the conserved Cys residues of the disulfide bond (Williams et al., Annu. Rev. Immunol. 88:381-405 (1988)). This finding supports the notion that introns may have been subsequently inserted to interrupt the ancestral Ig-like domain at later points during the evolution of each of these domains.

Expression of HB15 appears to be generally restricted to lymphocytes since two monoclonal antibodies reactive with HB15 failed to detect HB15 on most other hematopoietic cells. HB15 expression may be a late event in lymphocyte development as most thymocytes and circulating lymphocytes did not express detectable levels of cell surface HB15. However, after being activated by mitogens, peripheral lymphocytes expressed maximal levels of cell surface HB15 on days 3 through 5, the period of time during which maximal proliferation occurred. HB15 may be expressed at low levels by monocytes, especially after culture or activation, but the level of expression is low and may just result from Fc receptor mediated antibody attachment. Many T exd B cell lines also expressed HB15, but expression was generally at low levels. Interestingly, cell-surface HB15 expression by cell lines was highest during periods of maximal proliferation such as on the first day after the cultures were fed. These results imply that HB15 is important for maximal growth of lymphoblastoid cells or the maximal growth of cells is critical for the expression of this antigen. This was consistent with the observation that HB15 was expressed by germinal center cells in hematopoietic tissues. Nevertheless, HB15 expression appeared to be lymphoid tissue restricted as revealed by immunohistological analysis of twenty-two different tissues. The only exception was the finding that skin Langerhans cells express HB15. This unique pattern of restricted expression, along with the structural analysis of the protein, indicates that HB15 is a newly identified lymphocyte activation antigen.

The structural similarity of HB15 with other members of the Ig superfamily suggests that it may be involved in cellular interactions, since Ig-like domains are frequently involved in a variety of homotypic and heterotypic interactions in the immune and nervous systems. These interactions include binding functions that trigger a subsequent event below the cell surface or adhesion. A key functional feature is that homophilic or heterophilic binding usually occurs between Ig-related molecules, and this is often between molecules on opposed membrane surfaces. The structural relatedness of HB15 to these other proteins may imply a role for this lymphocyte activation protein in either homotypic or heterotypic interactions of lymphocytes following activation of other HB15⁺ cell types. As used herein, "homophilic" refers to cells of the same type that have a specific association or attraction for each other; "homotypic" refers to two molecules or cells of the same form that interact in a specific fashion; "heterophilic" refers to cells of different types having a specific association with each other; and "heterotypic" refers to two molecules or cells of different types that interact in a specific fashion.

It is understood that the particular nucleotide (SEQ ID NO:1) and amino acid (SEQ ID NO:3) sequences disclosed in FIG. 2 are representative of the human counterpart, and that related mammalian genes and their encoded proteins can be obtained following the teachings of this disclosure, as demonstrated herein for isolation of the mouse HB15 homolog. A mammalian homolog of the sequences disclosed in FIG. 2 will include a gene which is identified under stringent hybridizations conditions using a probe based on an approximately 20 nucleotide region of sequence identity between the FIG. 2 nucleotide sequence and the gene encoding the mammalian homolog. For example, cross-hybridization of the disclosed nucleic acid sequences with genetic material from human cells, can readily be performed to obtain equivalent human sequences; for example, see the oligonucleotide sequences presented in Table 1. In an analogous manner, degenerate oligonucleotides can readily be synthesized from the disclosed amino acid sequence, or portions thereof, and amplified using any well-known amplification technique, such as the polymerase chain reaction, to obtain probes that bind to equivalent human sequences. Proteins or polypeptides encoded by equivalent sequences can be produced. Antibodies directed against the disclosed protein or peptides can also be raised and employed to cross-react with human and other mammalian peptides having similar epitope(s). Those peptides isolated in this manner that have similar antibody reactivity patterns to those of the disclosed proteins or peptides are considered equivalents of the disclosed proteins or peptides.

The following examples are presented to illustrate the advantages of the present invention and to assist one of ordinary skill in making and using the same. These examples are not intended in any way otherwise to limit the scope of the disclosure.

EXAMPLE I

Human cDNA clones encoding HB15 were isolated and the encoded human HB15 protein characterized, as follows.

A human tonsil cDNA library was screened by differential hybridization (see Tedder et al., Proc. Natl. Acad. Sci., USA 85:208, 1988) , hereby incorporated by reference using labeled cDNA from the B lymphoblastoid cell line Raji and the T cell line H-SB2. Two of the 261 RAJI⁺ H-SB2⁻ cDNA clones isolated, pB10 (˜2.5 kb) and pB123 (˜1.2 kb), cross hybridized, yet failed to hybridize with cDNA that encode known B cell surface antigens (Tedder et al., supra).

Expression of the mRNA was examined by Northern blot analysis using poly(A)⁺ RNA isolated from B cell lines (NALM-6, Namalwa, Daudi, SB, and Raji), T cell lines (Hut-78, H-SB2, and MOLT-3) and the erythroleukemia line, K562. Poly(A)⁺ RNA was isolated as described (Maniatis et al., Molecular Cloning: A Laboratory Manual, (1982)). For Northern-blot analysis, 2 μAg of poly(A)⁺ RNA was denatured with glyoxal, fractionated by electrophoresis through a 1.1% agarose gel and transferred to nitrocellulose (Thomas, Methods Enzymol. 100:255 (1983)). The pB123 CDNA insert used as probe was isolated, nick-translated (Rigby et al., J. Mol. Biol. 113:237-251 (1977)) and hybridized with the filters as described (Wahl et al., Proc. Natl. Acad. Sci., USA 76:3683-3687 (1979)). Hybridization at high stringency was with 50% (v/v) formamide, 4×SSC, 10% (w/v) Na dextran sulfate at 42° C. The filters were washed at 65° C. with 0.2×SSC, 0.1% SDS. RNA size was determined by comparison with 28S and 18S ribosomal RNA run on the same gels as standards. The same blot was also hybridized with CDNA clones containing a housekeeping mRNA of unknown identity revealing that all mRNA were intact and were similar in quantity of this expressed mRNA. For hybridization at low stringency the conditions are overnight incubation at 42° C. in a solution comprising: 20% formamide, 5×SSC (150 mM NaCl, 15 mM trisodium citrate), 50 mM sodium phosphate (pH 7.6), 5×Denhardts solution, 10% dextran sulfate, and 20 μg/ml denatured, sheared salmon sperm DNA.

The pBl23 cDNA hybridized strongly with three mRNA species of ˜1.7, ˜2.0 and ˜2.5 kb in SB and Raji. Daudi and Namalwa cells expressed lower levels of this mRNA. Further autoradiography of the blot (7 days) revealed that the NALM-6, Hut-78 and MOLT-3 cells also expressed these three mRNA species, but at much lower levels, and faint hybridization with H-SB2 RNA was detected. These results suggested differential expression of this gene among leukocyte subpopulations.

Restriction maps were generated for these cDNA, as described by Maniatis et al., Molecular Cloning: A Laboratory Manual, 1982, Cold Spring Harbor Press, CSH, N.Y., and their nucleotide sequences determined as described Sanger et al., Proc. Nat. Aca. Sci. 74:5463, 1977. Both cDNA were overlapping and contained open reading frames at their 5' ends with the pB123 cDNA having the longest 5' sequence. Since neither clone contained a translation initiation site, the pB10 cDNA insert was used to isolate 13 additional cross-hybridizing cDNA from a human tonsil library. The pB10 insert was purified, labeled by nick translation (Rigby et al., J. Mol. Biol. 113:237-251 (1977)) and used to isolate homologous cDNA by again screening the same human tonsil cDNA library in λgt11 (Weis et al., Proc. Natl. Acad. Sci., USA 83:5639-5643 (1986)) as described (Zhou et al., Immunogenetics 35:102-111 (1992)). Positive plaques were isolated, cloned and the CDNA inserts were removed by EcoR I digestion and subcloned into pSP65 (Melton et al., Nucleic Acids Res. 12:7035-7056 (1984)). Restriction maps and nucleotide sequence determination indicated that 12 of the cDNA were overlapping, with one cDNA having the longest sequence at the 5' end. The restriction map and nucleotide sequence of this clone, termed pHB15, is shown in FIG. 1. The full length cDNA clone is likely to include an ˜500 bp fragment at the 3' end that was removed from the CDNA by EcoR I digestion and subcloning. Eight other independent CDNA clones had similar EcoR I generated fragments and an EcoR I site was located at the identical nucleotide position in all cDNA that were sequenced.

The pHB15 cDNA had a 625 bp open reading frame, with the major portion of the cDNA representing untranslated sequence. The determined nucleotide sequence (SEQ ID NO:1) and predicted amino acid sequence (SEQ ID NO:2) of HB15 are given in FIG. 2. The predicted cleavage site used to generate the mature protein is shown by a vertical arrow. The numbers shown above the amino acid sequence designate amino acid residue positions of the putative mature protein and the numbers on the right designate nucleotide positions. Amino acids are designated by the single-letter code, and * indicates the termination codon. Nucleotides delineating translated regions with hydrophobic character are underlined. Amino acids indicating potential N-linked glycosylation attachment sites are underlined. A poly(A) attachment signal sequence is indicated by wavy underlining. The Cys residues are circled and amino acids which are often conserved in Ig-like domains are indicated by (+). Arrow heads below the nucleotide sequence denote exon/intron boundaries identified in another DNA clone.

The first ATG shown is the most likely initiation codon for translation since it conforms to the proposed translation initiation consensus sequence, (A/G)CCAUG (Kozak, Cell 44:283-292 (1986)). It is likely that the different mRNA species result from differential use of poly(A) attachment sites, AATAAA, since one was found at nucleotide position 1248 in the middle of the 3' untranslated region (FIG. 2). This poly(A) attachment site was functional in the pBl23 CDNA since it was followed by a poly(A) tail. A poly(A) attachment site or tail was not found in the ˜550 bp EcoR I fragment which presumably represents the 3' end of the pHB15 cDNA.

One clone isolated from the cDNA library (˜3.0 kb long) that hybridized with the pB123 cDNA had a unique sequence with 229 and 107 bp long segments that were identical to those found in the other cDNA. These regions had flanking sequences that corresponded to the consensus 5' and 3' splice sequences which demark exon boundaries (Aebi et al., Trends Genet. 3:102-107 (1987)) indicating that this aberrant CDNA was composed of introns and two exons. The three splice junction sites identified by this clone are shown (FIG. 2).

The predicted length of the HB15 protein was 205 amino acids (FIG. 2). However, the pB123 cDNA was missing the codon AAG at nucleotide position 500 so the protein may be one amino acid shorter in some cases. This may result from differential splicing at an exon/intron border, that results in the inclusion or loss of a codon since this codon abuts a potential splice site. A similar phenomenon has been found in the CD19 gene which also encodes a member of the Ig superfamily (Zhou et al., Immunogenetics 35:102-111 (1992)). Hydropathy analysis of the HB15 amino acid sequence by the method of Kyte et al., J. Mol. Biol. 157:105 (1982) revealed two regions of strong hydrophobicity. The first hydrophobic stretch of 19 amino acids represents a typical signal peptide at the amino terminal end of the protein. The algorithm of von Heijne, Nucleic Acids Res. 14:4683-4690 (1986) predicts that the most probable amino-terminus of the mature protein would be the Thr following amino acid 19. The second hydrophobic region of 22 amino acids most probably represents the transmembrane region. Three potential N-linked glycosylation attachment sites (N-X-S/T) were found in the extracellular domain. Therefore, the predicted molecular mass of the core protein would be ˜20,500.

Six Cys residues were found in the extracellular domain of HB15 and one in the putative membrane spanning domain. One pair of these residues at positions 16 and 88 delineate Ig-like domains (Williams et al., Annu. Rev. Immunol. 88:381-405 (1988)). This domain contained many of the hallmark amino acids which define the V set of Ig-like domains. A computer search of nucleotide and protein sequences was conducted using the Protein Identification Resource Data (GenBank release 66 and Swiss-Prot-16). Gap penalties of -1 were assessed during sequence homology analysis for each nucleotide or amino acid in the sequence where a gap or deletion occurred. The computer search of protein sequences showed that no proteins shared significant sequence homology with HB15 other than some members of the Ig superfamily.

Referring to FIG. 3, a hypothetical model is given for the structure of the extracellular domain of HB15 based on the proposed arrangement of the β-pleated sheets for the V domain of Ig heavy chain. Cys residues are represented as filled circles and amino acids encoded by different exons are indicated by alternatively shaded circles. Numbers represent the predicted amino acid residue positions as in FIG. 2.

EXAMPLE II Preparation of HB15 Truncated and Chimeric Molecules

Variant forms of HB15, e.g., truncated molecules or chimeric (i.e., hybrid) molecules containing substituted domains, may be prepared using conventional recombinant DNA techniques known to those of skill in the art and the HB15 nucleotide sequences (SEQ ID NO:1--human and SEQ ID NO:3--mouse and amino acid sequences (SEQ ID NO:2--human and SEQ ID NO:4--mouse) disclosed herein. See Maniatis et al., 1982, Molecular Cloning, Cold Spring Harbor Laboratory, Cold Spring Harbor, N.Y., hereby incorporated by reference.

A chimeric HB15 molecule is one in which one or two of the extracellular, transmembrane, and cytoplasmic domains is removed and replaced by the corresponding domain from another species, e.g., a domain from the mouse sequences disclosed herein.

A truncated HB15 molecule is one in which a portion of the molecule has been deleted. Truncated molecules will include those molecules in which one or both of the transmembrane and cytoplasmic domains has been deleted from the molecule, leaving, minimally, the extracellular domain or a portion thereof. A truncated HB15 molecule may be used to construct a protein in which the truncated HB15 end is fused to an effector molecule such as a drug toxin, or imaging agent using conventional methods for joining such molecules at the DNA or polypeptide level.

For example, a truncated form of HB15 may include the extracellular domain free of the cytoplasmic and transmembrane domains. This representative truncated HB15 molecule may be constructed by cleaving a DNA fragment containing a nucleotide sequence encoding the extracellular domain using standard PCR amplification to amplify that region. The amplified fragment then may be ligated to compatible ends of an expression vector and transfected into a host cell, e.g., an activated lymphocyte, which allows for production of the encoded domain. Truncated molecules containing other portions of the HB15 molecule may be constructed using conventional PCR amplification procedures. One or more of these sites may be utilized, depending upon which domains of the HB15 molecule are preferred.

Chimeric forms of HB15 also may be constructed using conventional recombinant DNA technology and the nucleotide (SEQ ID NO:1--human and SEQ ID NO:3--mouse) and amino acid sequences (SEQ ID NO:2--human and SEQ ID NO:4--mouse) disclosed herein. For example, where a chimeric molecule comprising human extracellular and transmembrane HB15 domains and a murine cytoplasmic domain is desired, the human domains may be isolated using restriction enzymes which generate those portions of human HB15 and joined to a murine cytoplasmic domain using cloning techniques, and expressed as described above for truncated molecules.

EXAMPLE III Isolation of Mammalian Homolog of HB15

A nucleotide sequence encoding HB15 from another mammalian species may be isolated by first hybridizing a nucleic acid probe with a population of nucleic acid molecules from an animal species other than human under hybridization conditions sufficient to allow for annealing of the probe to a homologous region of the target gene.. The nucleic acid probe may encode full-length human HB15 or a fragment thereof; the encoded polypeptide will be recognizable by a monoclonal antibody specific for an HB15 determinant, and will be able to hybridize to a nucleic acid encoding a polypeptide having the amino acid sequence shown in SEQ ID NO: 2. The probe will thus identify a first nucleic acid molecule to which the probe preferably stringently hybridizes. The first nucleic acid molecule then may be isolated and will thus encode a polypeptide having an amino acid sequence shown in SEQ ID NO. 2.

If a partial HB15 molecule, e.g., a heterologous domain is isolated in lieu of an entire HB15 molecule, a second nucleic acid molecule to which the nucleic acid probe preferably stringently hybridizes may be identified and isolated, wherein the first and second nucleic acid molecules, joined together in an amino acid reading frame, encode an amino acid sequence of SEQ ID NO. 2.

Alternative strategies may also be used for isolating a mammalian HB15 homolog. For example, the mouse HB15 homolog was isolated as follows.

The mouse HB15 gene was isolated by screening a murine genomic library by cross-hybridization with a 1.7 kb subclone of the human HB15 CDNA under low stringency conditions.

Genomic DNA clones were isolated from a genomic DNA library made with partial Mbo I-digested mouse genomic DNA that was isolated from a 129Sv mouse strain and inserted into the vector lambda-DASH II (Stratagene, La Jolla, Calif.). The human HB15 cDNA clone was labeled by nick translation and used to screen the mouse genomic DNA library according to the method of Benton and Davis (1977, Science 196:180). Hybridization was performed at 42° C. in the presence of 30% formamide and the filters were finally washed at 50° C. in 1×SSC with 0.1% SDS (w/v). The human HB15 cDNA probe contained the entire protein coding sequence and the entire 3' untranslated regions. Positive plaques were isolated, and phage DNA were characterized by restriction enzyme mapping as described (Maniatis et al., 1982, Molecular Cloning, supra). DNA fragments of these clones were generated by EcoR I or Hind III digestions and were subcloned into the plasmids pSP65 or pSP64. Detailed restriction enzyme maps of the subclones were made. Exons were located by Southern hybridization analysis of endonuclease digested mouse genomic DNA clones using labeled human cDNA or oligonucleotide probes. Nucleotide sequences were determined by the dideoxy chain termination method (Sanger et al., 1977, Proc. Nat. Aca. Sci. 74:5463).

Overlapping mouse genomic clones spanning 23 kb contained most of the mouse HB15 gene, from the 3' half of the immunoglobulin domain through the 3' untranslated region. Sequence analysis of the 3' portion of the immunoglobulin-like domain, the transmembrane region, and the cytoplasmic domain demonstrated a significant degree of conservation between human and mouse sequences, such that amino acid identity is ˜70% in these exons (FIG. 6). Likewise, the 3' untranslated region contained 1600 bp of extremely well conserved nucleotide sequence.

FIG. 6 shows the nucleotide sequence of mouse HB15 (m) (SEQ ID NO:3) compared with the human (h) cDNA sequence (SEQ ID NO:1). The precise nucleotide sequence for the 5' region of the mouse HB15 protein is not definitive as indicated by nucleotides in lower case print. "*" indicates identity in nucleotide sequences between human and mouse. "-" indicates gaps in the nucleotide sequence introduced to generate the highest levels of homology. The predicted cleavage site used to generate the mature protein is shown by a vertical arrow. The numbers shown above the amino acid sequence designate amino acid residue positions of the mature human protein and the numbers on the right designate nucleotide positions for the human cDNA. Nucleotides delineating translated regions with hydrophobic character (leader and transmembrane domains) are double underlined. Amino acids indicating potential N-linked glycosylation attachment sites are underlined. A poly(A) attachment signal sequence is indicated by wavy underlining. Amino acids which are often conserved in lg-like domains are indicated by (+). Arrow heads below the nucleotide sequence denote exon/intron boundaries identified in genomic DNA clones.

The 5' portion of mouse HB15 was isolated by PCR amplification of a mouse B lymphocyte CDNA library using a 5' oligonucleotide sense probe homologous with the flanking sequence of the vector and using a 3' antisense oligonucleotide probe (#2489 in Table 1) homologous to the 5' half of the Ig like domain of mouse HB15. This generated an approximately 400 bp cDNA fragment that was subcloned and sequenced. The nucleotide sequence of the PCR product revealed that it was nearly identical in sequence to the human HB15 cDNA (FIG. 6). RNA was isolated by a modification of the single step acid-guanidine-phenol-chloroform method from the mouse B cell line A20. One microgram of this RNA was used to synthesize cDNA using random hexamer primer oligonucleotides and SUPERSCRIPT reverse transcriptase (Bethesda Research Laboratories). The cDNA synthesis reaction mixture contained 10 mM Tris-HCl (pH 8.3), 50 mM KCl, 1.5 mM MgCl₂, and 0.8 mM each of DATP, dGTP, dCTP, and dTTP (Sigma, St. Louis, Mo.). 500 ng of the hexamer primer, 200 U of reverse transcriptase, and 1 μl of RNasin (Promega) were added to give a final volume of 25 μl. After 1 hour at 37° C. this reaction mixture was stopped by heating to 95° C. for 5 min and then cooled to 4° C. for 5 min. 5 μl of this reaction mixture was used to perform polymerase chain reactions (PCR) by adding 10 μl of PCR buffer, 50 pmol sense and antisense primers and amplification was carried out for 30 cycles as follows: denature for 1 min. at 94° C., anneal for 1 min. at 55° C. and extend for 1 min. at 72° C.

The PCR amplified cDNAs were electrophoresed through 1% agarose gels and transferred to nitrocellulose. DNA size was determined by co-electropheresis of a 1-kb ladder (Bethesda Research Laboratories, Gaithersberg, Md.). Hybridization was performed at 50° C. in buffer containing a 5' end-labeled oligonucleotide, 6×NET (3M NaCl, 0.02 mM EDTA, 0.15 mM Tris-HCl ph 8.3), 10×Denhardt's solution, 0.1% SDS (w/v), 20 mM sodium phosphate, and 100 μg/ml salmon sperm DNA (Sigma). Filters were finally washed in 2×SSC at room temperature. Autoradiography was at room temperature for 30 min.

Within the immunoglobulin-like domain of human and mouse HB15, all cysteine residues have been conserved, including those which delineate the immunoglobulin-like domain in the human protein. Partial determination of intron/exon boundaries for the mouse HB15 gene has confirmed that, as with the human HB15 gene, the immunoglobulin-like domain in the mouse is encoded by at least two exons.

Mouse HB15 sequence-specific oligonucleotide primers generated from a portion of the immunoglobulin-like domain (#2406 in Table 1) and from the cytoplasmic domain (#2407 in Table 1) have been used as probes to examine the pattern of expression of HB15 in mouse. The presence of HB15-specific mRNA in spleen, kidney, liver, brain, muscle, lung, thymus, and thyroid tissue was tested by reverse transcriptase PCR and generated the expected DNA products in all organs. The identification of HB15 mRNA in multiple organ sites may reflect the presence of dendritic cell family members present as a network of supportive or accessory cells in diverse tissue types throughout the body.

HB15 cDNAs were isolated from niRNA as follows. cDNA was produced from Raji MRNA to determine whether oligonucleotides representing different domains of the molecule (FIG. 7) could be used as probes to generate HB15 nucleotide sequences. In FIG. 7, locations of oligonucleotides used for PCR amplification of cDNA. Oligonucleotides identical to the human sequence are shown above the human cDNA while oligonucleotides identical to the mouse sequence are below the human cDNA sequence. The 5' end of the oligonucleotide is indicated by an arrowhead; > for sense primers and < for antisense primers. cDNA was amplified by PCR and the resulting products were characterized by Southern blot analysis with probes that would hybridize with internal HB15 sequence. Both the entire open reading frame and the 5' and 3' ends of cDNAs were amplified using the strategy shown in FIGS. 8A and 8B. In FIG. 8A, HB15 cDNAs were generated from RNA isolated from the Raji B cell line and the cDNAs were amplified using appropriate combinations of a sense oligonucleotide and antisense oligonucleotide, whose sequences are defined in Table 1 as follows: 1. #2083 and 2407; 2. 2406 and 2407; 3. 2085 and 2407; 4. LJZ001 and 2086; 5. LJZ001 and 2489; 6. LJZ001 and 2084; 7. LJZ001 and TFT617; 9. LJZ001 and 2407. This strategy generated cDNA fragments representing the 5' end or 3' end of the HB15 coding region. FIG. 8A shows representative results from one experiment showing the PCR amplified cDNAs obtained; PCR-generated cDNAs were electrophoresed on an agarose gel with DNA size markers and stained with ethidium bromide. In FIG. 8B, Southern blots of replicates of the gel in A were probed with the end-labeled #2082 oligonucleotide. Autoradiographic results are shown. There were additional bands variably observed in some PCR reactions, but these bands were also seen in control reactions carried out with mRNA from HB15 negative cell lines (data not shown). These bands also failed to hybridize with internal ³² P-labeled oligonucleotide probe in most cases. Therefore, it is most likely that these minor species of PCR products represented artifact DNA generated in the PCR amplification process and do not represent real MRNA species.

                                      TABLE 1                                      __________________________________________________________________________         Human/                                                                     Probe                                                                              Mouse                                                                              Orientation                                                                          Domain                                                                              Sequence                  SEQ ID NO:                        __________________________________________________________________________     LJZ001                                                                             m   sense Leader                                                                              GCC ATG TCG CAA GGC CTC CAG CTC C                                                                        5                                 2086                                                                               h   antisense                                                                            5'lg exon                                                                           AC ACG GTC TCC TGG GTC AAG                                                                               6                                 2084                                                                               h   antisense                                                                            3'lg exon                                                                           AC CTA AGT GGC AAG GTG ATC                                                                               7                                 2085                                                                               h   sense 3'lg exon                                                                           GA CAG CAC TAT CAT CAG AAG                                                                               8                                 2406                                                                               m   sense 3'lg exon                                                                           C TGC AGC TCG GGC ACC TAC AGG TG                                                                         9                                 2489                                                                               m   antisense                                                                            3'lg exon                                                                           C TGC AGC TCG GGC ACC TAC AGG TG                                                                         10                                2083                                                                               h   sense TM exon                                                                             T GCA CAG CGT AAA GA      11                                LJ33                                                                               h   antisense                                                                            TM exon                                                                             ACT TTT AAG AAA TAC AGA GCG GAG ATT GTC                                                                  12                                TFT617                                                                             h   antisense                                                                            TM exon                                                                             G AAA TAC AGA GCG GAG ATT GTC CT                                                                         13                                2082                                                                               h   antisense                                                                            TM exon                                                                             ACA CTC ATC ATT TTC ACT TGT                                                                              14                                2407                                                                               m   antisense                                                                            cyto.tall                                                                           A GCT TTT CTT CCA GTC ACC TCC CCA A                                                                      15                                __________________________________________________________________________

EXAMPLE IV Production of monoclonal antibodies reactive with HB15

A monoclonal antibody reactive with HB15 or an HB15 homolog or portion thereof, particularly a portion of the extracellular domain of the molecule, may be prepared as described below for preparation of the anti-HB15a and anti-HB15b antibodies.

1. Preparation of Anti-HB15a and Anti-HB15b Antibodies

Hybridomas were generated by the fusion of NS-1 myeloma cells with spleen cells obtained from mice immunized with pHB15 cDNA-transfected COS cells. COS cells were transfected with the pHB15 cDNA insert subcloned into a modified CDM8 vector (Aruffo et al., EMBO J. 6:3313 (1987); Tedder et al., J. Immunol. 143:712-717 (1989)) using the DEAE-dextran method as described (Aruffo et al., EMBO J. 6:3313 (1987)). Cell surface expression was examined after 48 hours by indirect immunofluorescence. Stable cDNA transfected cells were produced using the pHB15 cDNA cloned into the BamH I site of the retroviral vector PZIPHEESV(X) in the correct orientation (Cepko et al., Cell 37:1053-1062 (1984)). The murine pre-B cell line, 300.19, and the human erythroleukemia cell line, K562, were transfected with this vector by electroporation with subsequent selection of stable transfectants using G418 (Gibco/BRL). Cells expressing HB15 were further enriched by reacting the cells with monoclonal antibodies with the subsequent isolation of HB15⁺ cells by panning on anti-mouse Ig coated plates. Cell lines were grown in RPMI 1640 medium containing 10% fetal calf serum and antibiotics. Cultures of all cell lines were split the day before analysis and were in logarithmic growth.

Anti-HB15 mAb were generated as described (Tedder et al., J. Immunol. 144:532-540 (1990)) by the fusion of NS-1 myeloma cells with spleen cells from BALB/c mice that were repeatedly immunized with COS cells transfected with the HB15 cDNA. Each hybridoma was cloned twice and used to generate ascites fluid. The isotypes of the mAb were determined using a mouse monoclonal antibody isotyping kit from Amersham (Arlington Heights, Ill.).

Monoclonal antibodies reactive in indirect immunofluorescence assays with HB15 MRNA positive cell lines, but not with HB15 negative cell lines, were isolated. Two of these antibodies, anti-HB15a (IgG_(2b)) and anti-HB15b (IgG₃) also reacted with COS cells transfected with the pHB15 cDNA, but did not react with cells transfected with CD19 cDNA (Tedder et al., J. Immunol. 143:712-717 (1989)) or the expression vector alone. In addition, these antibodies reacted with a human erythroleukemia cell line, K562, and a mouse pre-B cell line, 300.19, stably transfected with the pHB15 cDNA. The antibodies did not react with untransfected parent cells, cells transfected with vector alone; or CD19, CD20 (Tedder et al., Proc. Natl. Acad. Sci., USA 85:208 (1988)) or LAM-1 (Tedder et al., J. Exp. Med. 170:123-133 (1989)) cDNA transfected cells. In all cases, the reactivities of the anti-HB15a and anti-HB15b mAb were identical.

2. Mapping of HB15 Epitopes.

A monoclonal antibody specific for a given region of HB15 may be made using a peptide corresponding to the region of the molecule as an immunogen, and using conventional hybridoma production procedures. In addition, the cross-reactivity of such antibodies can be ascertained as follows. For example, the HB15a and HB15b mAb identify different epitopes on the HB15 molecule. The HB15a mAb was conjugated to FITC (HB15a-FITC). K562 cells transfected with the HB15 cDNA were first reacted with saturating amounts of either the HB15a or the HB15b mAb in the form of diluted ascites fluid. After the appropriate incubation period, the cells were subsequently washed and then treated with HB15a-FITC. After the appropriate incubation period, the cells were washed again to remove unbound HB15a-FITC and analyzed by fluorescence-based flow cytometry. Cells pretreated with HB15a mAb did not bind HB15a-FITC since the unlabeled mAb blocked the binding of the labeled reagent. In contrast, treatment of the cells with HB15b mAb had no effect on the staining of the test cells with the HB15a-FITC. These results demonstrate that the HB15a mAb binds to a different epitope of the HB15 molecule than the HB15b mAb.

Other HB15-reactive monoclonal antibodies may be produced using the amino acid sequence disclosed in SEQ ID NO:2, and portions thereof longer than 8-10 amino acids, using antibody production techniques described herein and in the literature.

For example, monoclonal antibodies to the protein or a fragment thereof may be made by the somatic cell hybridization techniques described initially by Kohler, B. and Milstein, C., Nature (1975) 256:495-497. The procedure involves immunizing a host animal (typically a mouse because of the availability of murine myelomas) with the protein. Antibody-producing cells (e.g., peripheral blood lymphocytes, splenocytes) are taken from the immunized host and mixed with a suitable tumor fusion partner in a liquid growth medium containing a fusogen such as polyethylene glycol of molecular weight 2000 to 5000. After the fusion the cells are washed to remove residual fusion medium and incubated in a selective growth medium (i.e., a growth medium containing additives to which the parent tumor line is sensitive) such as HAT medium. Surviving hybrids may be expanded and their culture media screened for the presence of antibodies by radioimmunoassay (RIA). Positive cultures may be screened for their ability to recognize and bind to the protein by immunoprecipitating labeled cell extracts with the positive cultures and analyzing the precipitate by SDS-PAGE for the presence of a labeled component. Hybrids that produce antibody that binds specifically to the protein may be subcloned and grown in vitro or in vivo by known procedures. The antibody may be isolated from the resulting culture medium or body fluid, as the case may be, by conventional procedures for isolating immunoglobulins.

Thus, monoclonal antibodies may be made against multiple epitopes of the HB15 polypeptide or an HB15 mammalian homolog.

EXAMPLE V Detection of HB15 expression

1. Immunoprecipitation of cell surface HB15

In order to detect the presence of HB15 or an HB15 homolog on certain cell types, an anti-HB15 monoclonal antibody may be used to immunoprecipitate the cognate antigen from a given cell type, as follows.

The anti-HB15a mAb was purified, coupled to beads and used to immunoprecipitate HB15 from detergent solubilized extracts of surface-iodinated cell lines, as follows. Cells were washed twice, resuspended in saline and labeled by the iodogen method as described (Thompson et al., Biochem. 26:743-750 (1987)). After washing, the cells were lysed in 1 ml of buffer containing 1% (v/v) TRITON X-100 and protease inhibitors as described (Tedder et al., Proc. Natl. Acad. Sci., USA 85:208 (1988)). Immunoprecipitations were carried out using anti-HB15a mAb or mouse Ig (as a negative control) directly conjugated to AFFIGEL (BioRad, Richmond, VA) at 2 mg of mAb per ml of gel according to the manufacturer's instructions. Cell lysates were precleared twice for 2 hours using 50 μl (50% v/v) of murine Ig coated beads at 4° C. Cell lysates were precleared again overnight. Half of the precleared lysate was then incubated with 25 μl of anti-HB15a mAb-coated beads or murine Ig-coated beads with constant rotation at 4° C. for 18 hours. Immunoprecipitates were washed and analyzed by SDS-PAGE as described (Tedder et al., Proc. Natl. Acad. Sci., USA 85:208 (1988)) with half of the sample run in the presence of 5% 2-mercaptethanol (reducing conditions). M_(r) were determined using pre-stained standard molecular weight markers (Gibco/BRL).

Optimum results were obtained using the K562-HB15 cell line (K562 cells transfected with pHB15 cDNA) since the level of HB15 expression was higher than in other cell lines. The anti-HB15a mAb specifically immunoprecipitated proteins that migrated as a single broad band of ˜45,000 M_(r). Similar results were obtained when the immunoprecipitated materials were run under reducing or nonreducing conditions. A similar protein was immunoprecipitated from the Raji cell line except the M_(r) was ˜40,000. Thus, HB15 was expressed as a noncovalently-associated single chain molecule on the cell surface.

2. HB15 is expressed by activated lymphocytes

In order to determine the tissue distribution of HB15 or an HB15 homolog, an anti-HB15 monoclonal antibody may be used to identify the presence of the cognate antigen by immunofluorescence staining and/or immunohistological analysis of different tissues, as follows. Cells were kept at 4° C. and were examined immediately after isolation. Indirect immunofluorescence analysis of viable cells was carried out after washing the cells three times. The cells were then incubated for 20 min on ice with each mAb as ascites fluid diluted to the optimal concentration for immunostaining. Isotype-matched murine antibodies that were unreactive with human leukocytes were used as negative controls. After washing, the cells were treated for 20 min at 4° C. with fluorescein isothiocyanate-conjugated goat anti-mouse Ig antibodies (Southern Biotechnology Associates, Birmingham, Ala.). Single color immunofluorescence analysis was performed on an EPICS PORFILE flow cytometer (Coulter Electronics, Hialeah, Fla.). Ten thousand cells were analyzed for each sample. All tissues were stained applying a modification of the APAAP procedure as described by Cordell et al., J. Histochem. Cytochem. 31:219-229 (1984). Basically, the slides were first incubated with monoclonal antibody followed by an incubation step with rabbit anti-mouse (bridging) antibody. Subsequently, a monoclonal antibody against alkaline phosphatase pre-incubated with alkaline phosphatase was applied. In order to enhance the sensitivity of this procedure, the number of phosphatase molecules on the surface was increased by using one or two layers of bridging antibody and anti-phosphatase antibody. Bound phosphatase molecules were visualized using new fuchsin as a substrate (Cordell et al., J. Histochem. Cytochem. 31:219-229 (1984)).

The tissue distribution of the HB15 surface antigen was examined by indirect immunofluorescence staining with flow cytometry analysis. Two cell lines that did not express HB15 message were transfected with the pHB15 cDNA subcloned into the Bam HI site of the retroviral vector PZIPNEVSV(X). Referring to FIG. 4, the immunofluorescence results obtained with three lymphoblastoid cell lines that express HB15 are demonstrated. The open histograms show the cellular reactivity with the HB15a antibody, and the shaded histograms demonstrate background levels of immunofluorescence staining obtained with unreactive control antibodies. Among 33 cell lines examined, HB15 was expressed at detectable levels by B cell lines (including Raji, Daudi, Namalwa, Arent, BJAB, SB, Jijoy, Akata, and SLA) and T cell lines (including Jurkat, H-9, Rex, H-SB2, and Hut-78). However, HB15 expression was generally low and variable. The highest levels of cell-surface expression were always obtained where the cell cultures were recently split and were thus proliferating maximally. Cell lines that did not express detectable levels of HB-15 included: K562; the B cell lines NALM-6 and Ramos; the T cell lines, MOLT-3, RPMI 8405, PEER, MOLT-14, CEM and HPB-ALL; the myelomonocytic line, HL60; the natural killer cell line, YT; the colon carcinoma lines, Colo-205 and HT29; the lung cell lines, NCI-H69, and NCI-H82, the prostate line, PC3; the melanoma line, MEWO; and the breast tumor lines, ZRT5.1, MCF7 and BT20.

Expression of HB15 by normal blood leukocytes was also examined. Human blood was obtained by protocols approved by the Human Protection Committee of Dana-Farber Cancer Institute and mononuclear cells were isolated by FICOLL HYPAQUE density gradient centrifugation. Mononuclear cells (10⁶ /ml) in complete media (RPMI-1640 supplemented with 15% fetal calf serum, antibiotics and glutamine) were stimulated with phytohemagglutinin-P (2 μg/ml; Difco, Detroit, Mich.), Con A (10 μg/ml, Miles Laboratories, Elkhart, Ind.), pokeweed mitogen (10 μg/ml, Gibco/BRL, Bethesda, Md.) or phorbol myristate 13-acetate (PMA, 10 ng/ml, Sigma, St. Louis, Mo.) as described (Tedder et al., J. Immunol. 144:532-540 (1990)). Lymphocytes were harvested at the indicated time points, washed once in complete media, and aliquoted for immediate immunofluorescence staining.

Cell-surface expression of HB15 was not detected at significant levels on circulating lymphocytes, natural killer cells or monocytes in 15 blood samples. Therefore, the possibility that HB15 was expressed following cellular activation was examined by inducing T lymphocyte proliferation with the mitogens concanavalin A (ConA), pokeweed mitogen, phytohemag-glutinin-P or phorbol esters (PMA). Expression of HB15 was examined 2, 8, 12, 24, 48, 72, 120 and 240 hours following the initiation of cultures. Appearance of HB15 expression paralleled cellular proliferation such that optimal expression was on days 3 through 5 following the initiation of cultures. Also, the quantity of HB15 expression induced was not correlated with any specific mitogen, but correlated more with the strength of the mitogenic signal such that cell-surface expression was predominantly found on the larger blast cells. Therefore, HB15 was expressed by lymphocytes following activation.

3. Immunohistological analysis of HB15 expression

The lymphocyte specificity and tissue distribution of HB15 was also examined by immunohistological analysis of different human tissues. Basically, the anti-HB15a mAb was used to stain thymus, tonsil, spleen, lymph node, kidney, renal pelvis and ureter, Fallopian tube, liver, pancreas, stomach, breast, lung, esophagus, skeletal muscle, skin, uterus, salivary gland, thyroid gland, adrenal gland, heart, appendix and colon. (Referring to FIGS. 5A-5F), in most cases, HB15 expression appeared lymphocyte specific in that no significant reactivity was observed in non-lymphoid tissues. Among tonsil and lymph nodes (FIG. 5A), HB15 was expressed reasonably strongly by scattered cells in intrafollicular regions (T cell zones) (FIG. 5C) . Although some of these cells may have been lymphoblasts, most were interdigitating reticulum cells (a subpopulation of dendritic cells) since they appeared larger than resting lymphocytes and expressed the CD1 surface molecule (FIG. 5D). Also, some cells (50-80%) within germinal centers (GC; FIGS. 5A and 5B) and follicular mantle zones (FM; FIG. 5A), with the morphology of lymphocytes, were weakly HB15⁺. Among spleen, the HB15⁺ cells were predominantly restricted to the white pulp, whereas the red pulp remained largely negative. Again, these large, scattered positive cells in the white pulp are likely to be interdigitating reticulum cells or lymphoblasts. Cortical thymocytes were HB15 negative, while a small subpopulation of medullary cells, presumably thymocytes, was positive (FIG. 5E). Unlike other non-hematopoietic tissues, analysis of skin revealed that some cells with the characteristic scattered branching morphology of Langerhans cells (a subpopulation of dendritic cells) expressed HB15 at detectable levels (FIG. 5F). Among all non-hematopoietic tissues, where inflammatory infiltrations were apparent, a few scattered lymphocytes were found to express HB15. It is also likely that circulating dendritic cells are HB15⁺, but because of their low frequency they were not readily detected. Similarly, it is also likely that the malignant counterparts of dendritic cells express HB15 and that this molecule can be used as a diagnostic marker for malignant cells as the L428 cell line, which is a neoplastic cell line that was derived from Hodgkin's disease and may represent interdigitating reticulum cells (Schaadt et al., Int. J. Cancer 26:723-731 (1980)), is HB15 positive.

It is to be understood that an HB15 homolog, like HB15 itself, will resemble HB15 in its tissue distribution pattern. That is, an HB15 homolog will be present on activated lymphocytes and generally absent on inactivated lymphocytes, although the presence or absence of the homolog on specific cell lines may not be directly correlated with the presence or absence of HB15 on such cell lines.

EXAMPLE VI Quantitation of HB15 Levels.

Endogenous levels of HB15 polypeptide or an HB15 polypeptide homolog in serum can be quantitated using the monoclonal antibodies that have been produced against HB15 according to any one of a number of quantitation methods known to those of ordinary skill in the art, including an enzyme-linked immunoassay (ELISA). For example, a serum sample may be obtained and serially diluted prior to analysis. The dilutions may be assayed in a conventional ELISA wherein the detecting antibody is an anti-HB15 antibody described herein. Detection and quantitation of HB15 in the serum sample are performed as described in art.

Uses

The HB15 protein or immunospecific fragments thereof, or antibodies or other antagonists to HB15 function, have a variety of uses, some of which are described below.

1. HB15 as a Marker for Non-follicular Dendritic Cells.

There are at present no specific markers for non-follicular dendritic cells in humans. Use of HB15 monoclonal antibody to identify HB15⁺ cells permits the isolation and purification of cells expressing this protein from a population of unrelated cells.

2. HB15 as a Marker for Cell Sarcomas and Malignant Phenotypes.

The HB15 monoclonal antibody will also be useful for evaluation and diagnosis of interdigitating cell sarcomas or other malignant cell types expressing this antigen. Therefore, HB15-based agents may be suitable for immunotherapy or immunoimaging. HB15 protein or immunospecific fragments thereof, or antibodies which antagonize HB15 function are useful for diagnosis or treatment of a variety of immunological disorders. For such purposes, the soluble external domain may be employed, typically but not necessarily, polymerized in a multivalent state using, e.g., dextran or polyamino acid carriers or fusion proteins of HB15 fragments and carrier molecules. Alternatively, liposomes may be employed as the therapeutic vehicle, in which case the transmembrane domain and preferably at least some of the cytoplasmic domain will also be included.

For example, since Langerhans cells are the primary immunocompetent cell in the skin, playing a role in the presentation of antigen to T cells and the induction of contact hypersensitivity, and since HB15 is expressed by Langerhans cells and may be involved in antigen presentation, it is likely to be involved in the pathogenesis of human skin disease such as psoriasis, autoimmune disorders, organ transplant and AIDS. Therefore, antagonists to HB15 function can provide important therapeutic agents for treatment of these diseases.

Similarly, since HB15 may serve as an accessory molecule for lymphocyte activation, the HB15 antigen, fragments or domains thereof, may be used as agonists that would augment or inhibit an immune response.

More specifically, the dendritic cell is a primary target of the human immunodeficiency virus, the causative agent of AIDS. It has recently been proposed that 80% of AIDS virus in vivo is produced by dendritic cells, particularly by Langerhans cells, circulating dendritic cells and interdigitating reticulum cells (Langhoff et al., Proc. Natl. Acad. Sci. USA 88:7998-8002 (1991)). Also, most infections occur through mucosal surfaces where it is thought that dendritic cells are first infected. Therefore, this reagent provides us with a critical tool for the potential prevention or treatment of AIDS or AIDS related disorders.

Certain clinical conditions may be monitored using in vitro assays to quantitate the levels of endogenous soluble HB15 in a patient's blood serum. Based on the finding that several receptors are now known to be shed during various normal and pathological conditions, it is possible that HB15 is also lost from the cell surface by an enzymatic process. Also, quantitative detection can be useful in a method of identifying leukocytes with abnormal or decreased expression of HB15 for diagnosis and/or detection of leukocyte activation or altered leukocyte function. Additionally, the ability to quantitate the amount of receptor, or fragment thereof, produced during the manufacture of a recombinant therapeutic agent will be advantageous for determining the efficacy of the agent.

Similarly, in treating certain clinical conditions, it may be advisable to remove endogenous soluble HB15 or HB15⁺ cells from a patient's blood. This can be done with existing on-line and off-line techniques by employing immunoselection columns containing antibodies or other binding agents directed against the disclosed external domain of HB15.

While the present invention has been described in conjunction with a preferred embodiment, one of ordinary skill, after reading the foregoing specification, will be able to effect various changes, substitutions of equivalents, and other alterations to the compositions and methods set forth herein. It is therefore intended that the protection granted by Letters Patent hereon be limited only by the definitions contained in the appended claims and equivalents thereof.

Deposits

The following hybridomas were deposited on March 17, 1992, with the American Type Culture Collection (ATCC) under the terms of the Budapest Treaty.

    ______________________________________                                         Identification        ATCC Designation                                         ______________________________________                                         Anti-HB15a Hybridoma cell line, HB15a                                                                HB 10987                                                 Anti-HB15b Hybridoma cell line, HB15b                                                                HB 10988                                                 ______________________________________                                    

Applicants' assignee, Dana-Farber Cancer Institute, Inc., represents that the ATCC is a depository affording permanence of the deposit and ready accessibility thereto by the public if a patent is granted. All restrictions on the availability to the public of the material so deposited will be irrevocably removed upon the granting of a patent. The material will be available during the pendency of the patent application to one determined by the Commissioner to be entitled thereto under 37 CFR 1.14 and 35 USC 122. The deposited material will be maintained with all the care necessary to keep it viable and uncontaminated for a period of at least five years after the most recent request for the furnishing of a sample of the deposited microorganism, and in any case, for a period of at least thirty (30) years after the date of deposit or for the enforceable life of the patent, whichever period is longer. Applicants' assignee acknowledges its duty to replace the deposit should the depository be unable to furnish a sample when requested due to the condition of the deposit.

    __________________________________________________________________________     SEQUENCE LISTING                                                               (1) GENERAL INFORMATION:                                                       (iii) NUMBER OF SEQUENCES: 15                                                  (2) INFORMATION FOR SEQ ID NO:1:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 2272 base pairs                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: NO                                                            (ix) FEATURE:                                                                  (A) NAME/KEY: CDS                                                              (B) LOCATION: 11..625                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:1:                                        GAATTCCGCCATGTCGCGCGGCCTCCAGCTTCTGCTCCTGAGCTGCGCC49                            MetSerArgGlyLeuGlnLeuLeuLeuLeuSerCysAla                                        1510                                                                           TACAGCCTGGCTCCCGCGACGCCGGAGGTGAAGGTGGCTTGCTCCGAA97                             TyrSerLeuAlaProAlaThrProGluValLysValAlaCysSerGlu                               152025                                                                         GATGTGGACTTGCCCTGCACCGCCCCCTGGGATCCGCAGGTTCCCTAC145                            AspValAspLeuProCysThrAlaProTrpAspProGlnValProTyr                               30354045                                                                       ACGGTCTCCTGGGTCAAGTTATTGGAGGGTGGTGAAGAGAGGATGGAG193                            ThrValSerTrpValLysLeuLeuGluGlyGlyGluGluArgMetGlu                               505560                                                                         ACACCCCAGGAAGACCACCTCAGGGGACAGCACTATCATCAGAAGGGG241                            ThrProGlnGluAspHisLeuArgGlyGlnHisTyrHisGlnLysGly                               657075                                                                         CAAAATGGTTCTTTCGACGCCCCCAATGAAAGGCCCTATTCCCTGAAG289                            GlnAsnGlySerPheAspAlaProAsnGluArgProTyrSerLeuLys                               808590                                                                         ATCCGAAACACTACCAGCTGCAACTCGGGGACATACAGGTGCACTCTG337                            IleArgAsnThrThrSerCysAsnSerGlyThrTyrArgCysThrLeu                               95100105                                                                       CAGGACCCGGATGGGCAGAGAAACCTAAGTGGCAAGGTGATCTTGAGA385                            GlnAspProAspGlyGlnArgAsnLeuSerGlyLysValIleLeuArg                               110115120125                                                                   GTGACAGGATGCCCTGCACAGCGTAAAGAAGAGACTTTTAAGAAATAC433                            ValThrGlyCysProAlaGlnArgLysGluGluThrPheLysLysTyr                               130135140                                                                      AGAGCGGAGATTGTCCTGCTGCTGGCTCTGGTTATTTTCTACTTAACA481                            ArgAlaGluIleValLeuLeuLeuAlaLeuValIlePheTyrLeuThr                               145150155                                                                      CTCATCATTTTCACTTGTAAGTTTGCACGGCTACAGAGTATCTTCCCA529                            LeuIleIlePheThrCysLysPheAlaArgLeuGlnSerIlePhePro                               160165170                                                                      GATTTTTCTAAAGCTGGCATGGAACGAGCTTTTCTCCCAGTTACCTCC577                            AspPheSerLysAlaGlyMetGluArgAlaPheLeuProValThrSer                               175180185                                                                      CCAAATAAGCATTTAGGGCTAGTGACTCCTCACAAGACAGAACTGGTA625                            ProAsnLysHisLeuGlyLeuValThrProHisLysThrGluLeuVal                               190195200205                                                                   TGAGCAGGATTTCTGCAGGTTCTTCTTCCTGAAGCTGAGGCTCAGGGGTGTGCCTGTCTG685                TTACACTGGAGGAGAGAAGAATGAGCCTACGCTGAAGATGGCATCCTGTTTTGAAGTCCT745                TCACCTCACTGAAAACATCTGGAAGGGGATCCCACCCCATTTTCTGTGGGCAGGCCTCGA805                AAACCATCACATGACCACATAGCATGAGGCCACTGCTGCTTCTCCATGGCCACCTTTTCA865                GCGATGTATGCAGCTATCTGGTCAACCTCCTGGACATTTTTTCAGTCATATAAAAGCTAT925                GGTGAGATGCAGCTGGAAAAGGGTCTTGGGAAATATGAATGCCCCCAGCTGGCCCGTGAC985                AGACTCCTGAGGACAGCTGTCCTCTTCTGCATCTTGGGGACATCTCTTTGAATTTTCTGT1045               GTTTTGCTGTACCAGCCCAGATGTTTTACGTCTGGGAGAAATTGACAGATCAAGCTGTGA1105               GACAGTGGGAAATATTTAGCAAATAATTTCCTGGTGTGAAGGTCCTGCTATTACTAAGGA1165               GTAATCTGTGTACAAAGAAATAACAAGTCGATGAACTATTCCCCAGCAGGGTCTTTTCAT1225               CTGGGAAAGACATCCATAAAGAAGCAATAAAGAAGAGTGCCACATTTATTTTTATATCTA1285               TATGTACTTGTCAAAGAAGGTTTGTGTTTTTCTGCTTTTGAAATCTGTATCTGTAGTGAG1345               ATAGCATTGTGAACTGACAGGCAGCCTGGACATAGAGAGGGAGAAGAAGTCAGAGAGGGT1405               GACAAGATAGAGAGCTATTTAATGGCCGGCTGGAAATGCTGGGCTGACGGTGCAGTCTGG1465               GTGCTCGTCCACTTGTCCCACTATCTGGGTGCATGATCTTGAGCAAGTTCCTTCTGGTGT1525               CTGCTTTCTCCATTGTAAACCACAAGGCTGTTGCATGGGCTAATGAAGATCATATACGTG1585               AAAATTCTTTGAAAACATATAAAGCACTATACAGATTCGAAACTCCATTGAGTCATTATC1645               CTTGCTATGATGATGGTGTTTTGGGGATGAGAGGGTGCTATCCATTTCTCATGTTTTCCA1705               TTGTTTGAAACAAAGAAGGTTACCAAGAAGCCTTTCCTGTAGCCTTCTGTAGGAATTCCT1765               TTTGGGGAAGTGAGGAAGCCAGGTCCACGGTCTGTTCTTGAAGCAGTAGCCTAACACACT1825               CCAAGATATGGACACACGGGAGCCGCTGGGCAGAAGGGACTTCACGAAGGTTTGCATGGA1885               TGTTTTAGCCATTGTTGGCTTTCCCTTATCAAACTTGGGCCCTTCCCTTCTTGGTTTCCA1945               AAGGCATTTTATTGCTTGAGTTATATGTTCACTGTCCCCCTAATATTAGGGAGTAAAACG2005               GATACCAAGTTGATTTAGTGTTTTTACCTCTGTCTTGGCTTTCATGTTATTAAACTGATG2065               CATGTGAAGAAAGGGTGTTTTTCTGTTTTATATTCAACTCATAAGACTTTGGGATAGGAA2125               AAATGAGTAATGGTTACTAGGCTTAATACCTGGGTGATTACATAATCTGTACAATGAACC2185               CCCATGATGTAAGTTTACCTATGTAACAAACCTGCACTTATACCCATGAACTTAAAATGA2245               AAGTTAAAAATAAAAAACATATACAAA2272                                                (2) INFORMATION FOR SEQ ID NO:2:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 205 amino acids                                                    (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:2:                                        MetSerArgGlyLeuGlnLeuLeuLeuLeuSerCysAlaTyrSerLeu                               151015                                                                         AlaProAlaThrProGluValLysValAlaCysSerGluAspValAsp                               202530                                                                         LeuProCysThrAlaProTrpAspProGlnValProTyrThrValSer                               354045                                                                         TrpValLysLeuLeuGluGlyGlyGluGluArgMetGluThrProGln                               505560                                                                         GluAspHisLeuArgGlyGlnHisTyrHisGlnLysGlyGlnAsnGly                               65707580                                                                       SerPheAspAlaProAsnGluArgProTyrSerLeuLysIleArgAsn                               859095                                                                         ThrThrSerCysAsnSerGlyThrTyrArgCysThrLeuGlnAspPro                               100105110                                                                      AspGlyGlnArgAsnLeuSerGlyLysValIleLeuArgValThrGly                               115120125                                                                      CysProAlaGlnArgLysGluGluThrPheLysLysTyrArgAlaGlu                               130135140                                                                      IleValLeuLeuLeuAlaLeuValIlePheTyrLeuThrLeuIleIle                               145150155160                                                                   PheThrCysLysPheAlaArgLeuGlnSerIlePheProAspPheSer                               165170175                                                                      LysAlaGlyMetGluArgAlaPheLeuProValThrSerProAsnLys                               180185190                                                                      HisLeuGlyLeuValThrProHisLysThrGluLeuVal                                        195200205                                                                      (2) INFORMATION FOR SEQ ID NO:3:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 2197 base pairs                                                    (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: NO                                                            (ix) FEATURE:                                                                  (A) NAME/KEY: CDS                                                              (B) LOCATION: 45..626                                                          (xi) SEQUENCE DESCRIPTION: SEQ ID NO:3:                                        ACCCACGCGTCCGCCCACGCGTCCGGTGTCGCAGCGCTCCAGCCATGTCGCAAGGC56                     MetSerGlnGly                                                                   CTCCAGCTCCTGTTTCTAGGCTGCGCTGCCTGGCACCGCGATGGCGAT104                            LeuGlnLeuLeuPheLeuGlyCysAlaAlaTrpHisArgAspGlyAsp                               5101520                                                                        GTGGAGGTGACGGTGGCTTGCTCCGAGACTGCCGACTTGCCTTGCACA152                            ValGluValThrValAlaCysSerGluThrAlaAspLeuProCysThr                               253035                                                                         GCGCCCTGGGACCCGCAGCTCTCCTATGCAGTGTCCTGGGCCAAGGTC200                            AlaProTrpAspProGlnLeuSerTyrAlaValSerTrpAlaLysVal                               404550                                                                         TCCGAGAGTGGCACTGAGAGTGTGGAGCTCCCGGAGAGCAAGCAAAAC248                            SerGluSerGlyThrGluSerValGluLeuProGluSerLysGlnAsn                               556065                                                                         AGCTCCTTCGAGGCCCCCAGGAGAAGGGCCTATTCCCTGACGATCCAA296                            SerSerPheGluAlaProArgArgArgAlaTyrSerLeuThrIleGln                               707580                                                                         AACACTACCATCTGCAGCTCGGGCACCTACAGGTGTGCCCTGCAGGAG344                            AsnThrThrIleCysSerSerGlyThrTyrArgCysAlaLeuGlnGlu                               859095100                                                                      CTCGGAGGGCAGCGCAACTTGAGCGGCACCGTGGTTCTGAAGGTGACA392                            LeuGlyGlyGlnArgAsnLeuSerGlyThrValValLeuLysValThr                               105110115                                                                      GGATGCCCCAAGGAAGCTACAGAGTCAACTTTCAGGAAGTACAGGGCA440                            GlyCysProLysGluAlaThrGluSerThrPheArgLysTyrArgAla                               120125130                                                                      GAAGCTGTGTTGCTCTTCTCTCTGGTTGTTTTCTACCTGACACTCATC488                            GluAlaValLeuLeuPheSerLeuValValPheTyrLeuThrLeuIle                               135140145                                                                      ATTTTCACCTGCAAATTTGCACGACTACAAAGCATTTTCCCAGATATT536                            IlePheThrCysLysPheAlaArgLeuGlnSerIlePheProAspIle                               150155160                                                                      TCTAAACCTGGTACGGAACAAGCTTTTCTTCCAGTCACCTCCCCAAGC584                            SerLysProGlyThrGluGlnAlaPheLeuProValThrSerProSer                               165170175180                                                                   AAACATTTGGGGCCAGTGACCCTTCCTAAGACAGAAACGGTA626                                  LysHisLeuGlyProValThrLeuProLysThrGluThrVal                                     185190                                                                         TGAGTAGGATCTCCACTGGTTTTTACAAAGCCAAGGGCACATCAGATCAGTGTGCCTGAA686                TGCCACCCGGACAAGAGAAGAATGAGCTCCATCCTCAGATGGCAACCTTTCGAAGTCCTT746                CACCTGACAGTGGGCTCCACACTACTCCCTGACACAGGGTCTTGAGCACCATCATATGAT806                CACGAAGCATGGAGTATCACCGCTTCTCTGTGCTGTCAGCTTAATGTTTCATGTGGCTAT866                CTGGTCAACCTCGTGAGTGCTTTTCAGTCATCTACAAGCTATGGTGAGATGCAGGTGAAG926                CAGGGTCATGGGAAATTTGAACACTCTGAGCTGGCCCTGTGACAGACTCCTGAGGACAGC986                TGTCTCTCCTACATCTGGGATACATCTCTTTGAATTTGTCCTGTTTCGTTGCACCAGCCC1046               AGATGTCTCACATCTGGCGGAAATTGACAGGCCAAGCTGTGAGCCAGTGGGAAATATTTA1106               GCAAATAATTTCCAGTGGCGAAGGTCCTGCTATTAGTAAGGAGTATTATGTGTACATAGA1166               AATGAGAGGTCAGTGAACTATTCCCCAGCAGGGCCTTTTCATCTGGAAAAGACATCCACA1226               AAAGCAGCAATACAGAGGGATGCCAGCATTTATTTTTTTAATCTTCATGTATTGTCAAAG1286               AAGAATTTTTCATGTTTTTTCAAAGAAGTGTGTTTCTTTCCTTTTTTAAAATATGAAGGT1346               CTAGTTACATAGCATTGCTACGTACAAGCAGCCTGAGAGAAGATGGAGAATGTTCCTCAA1406               AATAGGGACAGCAAGCTAGAACGACTGTACAGTGCCTGCTGGGAAGGGCAGACAATGGAC1466               TGAGAAACCAGAAGTCTGGCCACAAGATTGTCTGTATGATTCTGGACGAGTCACTTGTGG1526               TTTTCACTCTCTGGTTAGTAAACCAGATAGTTTAGTCTGGGTTGAATACAATGGATGTGA1586               AGTTGCTTGGGGAAAGCTGAATGTAGTGAATACATTGGCAACTCTACTGGGCTGTTACCT1646               GTTGATATCCTAGAGTTCTGGAGCTGAGACGATCGCTGTCATATCTCAGCTTGCCCATCA1706               ATCCAAACACAGGAGGCTACAAAAAGGACATGAGCATGGTCTTCTGTGTGAACTCCTCCT1766               GAGAAACGTGGAGACTGGCTCAGCGCTTTGTGCTCGAAGGACTAATCACAAGTTCTTCGA1826               AGATATGGACCTAGGGGAGCTATTGCGCCACGACAGGAGGAAGTTCTCAGATGTTGCATT1886               GATGTAACATTGTTGCATTTCTTTAATGAGCTGGGCTCCTTCCTCATTTGCTTCCCAAAG1946               AGATTTTGTCCCACTAATGGTGTGCCCATCACCCACACTATGAAAAGTAAAAGGGATGCT2006               GAGCAGATACAGGCTAGTCTTACCTCTCAAGTCCATGACTTTCATGCTATTAAAGAATGC2066               ATGTGAAGAGGTGTGTTCTTCTTTTCTATCTTTAAAATGATCGACTTTAGAGTGAGTGTT2126               TGGGTGCTGAGTGGAGAGTAAGAATGCAGAAATGGTAGTGGTAAATGACTGGCTGCTTCC2186               CGAGGGGATCC2197                                                                (2) INFORMATION FOR SEQ ID NO:4:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 194 amino acids                                                    (B) TYPE: amino acid                                                           (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: protein                                                    (xi) SEQUENCE DESCRIPTION: SEQ ID NO:4:                                        MetSerGlnGlyLeuGlnLeuLeuPheLeuGlyCysAlaAlaTrpHis                               151015                                                                         ArgAspGlyAspValGluValThrValAlaCysSerGluThrAlaAsp                               202530                                                                         LeuProCysThrAlaProTrpAspProGlnLeuSerTyrAlaValSer                               354045                                                                         TrpAlaLysValSerGluSerGlyThrGluSerValGluLeuProGlu                               505560                                                                         SerLysGlnAsnSerSerPheGluAlaProArgArgArgAlaTyrSer                               65707580                                                                       LeuThrIleGlnAsnThrThrIleCysSerSerGlyThrTyrArgCys                               859095                                                                         AlaLeuGlnGluLeuGlyGlyGlnArgAsnLeuSerGlyThrValVal                               100105110                                                                      LeuLysValThrGlyCysProLysGluAlaThrGluSerThrPheArg                               115120125                                                                      LysTyrArgAlaGluAlaValLeuLeuPheSerLeuValValPheTyr                               130135140                                                                      LeuThrLeuIleIlePheThrCysLysPheAlaArgLeuGlnSerIle                               145150155160                                                                   PheProAspIleSerLysProGlyThrGluGlnAlaPheLeuProVal                               165170175                                                                      ThrSerProSerLysHisLeuGlyProValThrLeuProLysThrGlu                               180185190                                                                      ThrVal                                                                         (2) INFORMATION FOR SEQ ID NO:5:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 25 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: NO                                                            (xi) SEQUENCE DESCRIPTION: SEQ ID NO:5:                                        GCCATGTCGCAAGGCCTCCAGCTCC25                                                    (2) INFORMATION FOR SEQ ID NO:6:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 20 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: YES                                                           (xi) SEQUENCE DESCRIPTION: SEQ ID NO:6:                                        ACACGGTCTCCTGGGTCAAG20                                                         (2) INFORMATION FOR SEQ ID NO:7:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 20 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: YES                                                           (xi) SEQUENCE DESCRIPTION: SEQ ID NO:7:                                        ACCTAAGTGGCAAGGTGATC20                                                         (2) INFORMATION FOR SEQ ID NO:8:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 20 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: NO                                                            (xi) SEQUENCE DESCRIPTION: SEQ ID NO:8:                                        GACAGCACTATCATCAGAAG20                                                         (2) INFORMATION FOR SEQ ID NO:9:                                               (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 24 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: NO                                                            (xi) SEQUENCE DESCRIPTION: SEQ ID NO:9:                                        CTGCAGCTCGGGCACCTACAGGTG24                                                     (2) INFORMATION FOR SEQ ID NO:10:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 24 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: YES                                                           (xi) SEQUENCE DESCRIPTION: SEQ ID NO:10:                                       CTGCAGCTCGGGCACCTACAGGTG24                                                     (2) INFORMATION FOR SEQ ID NO:11:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 15 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: NO                                                            (xi) SEQUENCE DESCRIPTION: SEQ ID NO:11:                                       TGCACAGCGTAAAGA15                                                              (2) INFORMATION FOR SEQ ID NO:12:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 32 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: YES                                                           (xi) SEQUENCE DESCRIPTION: SEQ ID NO:12:                                       ACTTTTAAGAAATACAGAGCGGAGATTGTCCT32                                             (2) INFORMATION FOR SEQ ID NO:13:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 24 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: YES                                                           (xi) SEQUENCE DESCRIPTION: SEQ ID NO:13:                                       GAAATACAGAGCGGAGATTGTCCT24                                                     (2) INFORMATION FOR SEQ ID NO:14:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 21 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: YES                                                           (xi) SEQUENCE DESCRIPTION: SEQ ID NO:14:                                       ACACTCATCATTTTCACTTGT21                                                        (2) INFORMATION FOR SEQ ID NO:15:                                              (i) SEQUENCE CHARACTERISTICS:                                                  (A) LENGTH: 26 base pairs                                                      (B) TYPE: nucleic acid                                                         (C) STRANDEDNESS: single                                                       (D) TOPOLOGY: linear                                                           (ii) MOLECULE TYPE: DNA (genomic)                                              (iii) HYPOTHETICAL: NO                                                         (iv) ANTI-SENSE: YES                                                           (xi) SEQUENCE DESCRIPTION: SEQ ID NO:15:                                       AGCTTTTCTTCCAGTCACCTCCCCAA26                                                   __________________________________________________________________________ 

What is claimed is:
 1. An antibody that specifically recognizes a polypeptide having the sequence of the extracellular domain, the transmembrane domain, or the cytoplasmic domain of the HB15 protein having the amino acid sequence given in SEQ ID NO:2.
 2. A monoclonal antibody that specifically recognizes the HB15 protein having the amino acid sequence given in SEQ ID NO:2.
 3. The monoclonal antibody of claim 2, said antibody being produced by the hybridoma cell line deposited as ATCC No. HB 10987, said antibody being denominated anti-HB15a.
 4. The monoclonal antibody of claim 2, said antibody being produced by the hybridoma cell line deposited as ATCC No. HB 10988, said antibody being denominated anti-HB15b.
 5. The hybridoma cell line deposited as ATCC No. HB
 10987. 6. The hybridoma cell line deposited as ATCC No. HB
 10988. 7. An imaging agent for imaging a site of HB15 expression in a human patient comprising a monoclonal antibody that specifically recognizes the HB15 protein having the amino acid sequence given in SEQ ID NO:2, said monoclonal antibody being labeled with a detectable label suitable for immunoimaging.
 8. An antibody that specifically recognizes an immunogenic polypeptide fragment having a sequence of a portion of the amino acid sequence disclosed in SEQ ID NO:2. 